Pipeline pigs for inspecting, servicing, and maintaining pipelines are typically moved along the interior of the pipeline under pressure. Absent any speed control, the pig travels at essentially the same rate of speed as the pipeline fluid propelling it. In many cases, the speed of the pipeline fluid exceeds the desired range in which the pig is most effective for its intended use. In addition, pipeline debris and build-up may slow the pig below a desired minimum speed, thereby requiring means for increasing pig speed. Therefore, various speed control means have been developed. Some are “passive” control devices, using only pressure flow. Others are “active” control devices, using electronics or electrical actuators.
Prior art passive speed control devices may be classified into two broad categories: valve-actuated control devices and passageway-adjusted control devices. Both categories of speed control devices work by altering the amount of bypass flow through the body of the pig. An example of an early valve-actuated control device is disclosed in U.S. Pat. No. 2,860,356. A hollow valve member having a front and rear set of ports is received by a sleeve located at a forward end of the pig. If the pig encounters resistance to its movement and slows, a pressure increase at the rearward end of the pig causes the valve member to move farther into the sleeve, thereby blocking the front ports and reducing flow through the valve and, therefore, increasing bypass flow though the pig. The reduced flow through the valve further increases the pressure differential across the pig and, therefore, the speed of the pig increases. By allowing more bypass flow through the pig when the pig stalls, debris may be flushed out of the way to allow the pig to start again. The valve, however, does not prevent the pig from stalling, nor does it prevent speed surges above that of the pipeline fluid propelling the pig.
Other valve-actuated control devices employ increasingly sophisticated active control means for operating the valve. None of those control means, however, prevent the pig from stalling or controlling speed surges. For example, U.S. Pat. No. 3,495,546 discloses the use of a valve that is opened or closed by a large piston coupled to a solenoid responsive to changes in line pressure. Actuation of the piston moves the valve, thereby controlling the degree of opening in a bypass port. A more sophisticated control means, such as that disclosed in U.S. Pat. No. 6,098,231 places a sleeve-type valve in communication with a microprocessor. The microprocessor monitors pig speed and, when speed falls outside a predetermined range, the microprocessor sends a signal to extend or retract a hydraulic actuator connected to the valve. By controlling the actuator, a portion of the sleeve is drawn over, or removed from, a set of circumferential exit ports, thereby increasing or decreasing flow through the pig.
As to the second category of speed control devices, passageway-adjusted, U.S. Pat. No. 4,769,598 provides a typical example. Two perforated discs rotatable in relation to one another are mounted external to the pig body in an annular space between the front sealing elements. The degree of alignment in the perforations in the two discs permits the speed of the pig to vary by allowing more or less fluid to flow through the interior passageway of the pig. Similar to the above patent is U.S. Pat. No. 6,190,090, which discloses the use of a first and second bonnet. Each bonnet has several openings and is mounted to a forward end of the pig. A stepper motor and controller vary the degrees of alignment in the bonnets and, therefore, regulate bypass flow through the pig.
Another example of an active passageway-adjusted device is found in U.S. Pat. No. 5,208,906, which discloses the use of a set of longitudinal passageways through the interior of the pig and a set of movable plates that adjust the flow through the passageways. The plates are typically adjusted by a step motor and controller which are, in turn, controlled by a comparator circuit that compares the actual speed of the pig with the desired speed.
With the exception of U.S. Pat. No. 2,860,356, all of the above speed control devices rely upon sophisticated control means for either varying the position of the valve or the alignment or size of the passageways to shunt bypass flow. U.S. Pat. No. 2,860,356, however, requires the use of multiple valves in large pigs and in high velocity pipeline applications; therefore, it—along with the other prior alt devices—is not optimized for maximum speed reduction of the pig. Additionally, none of the prior art devices prevent stalling or surging. When the pig stalls, significant pressure may build up behind the pig and cause the pig to surge to speeds even higher than the average fluid flow through the pipeline. This is problematic, for example, in dispersal-type pigs.
A dispersal pig is configured to move fluid (gas and liquid) forward in advance of the pig and includes one or more nozzles located at a forward end of the pig. Differential pressure across the pig allows the gas flow to draw liquid into the nozzle and a spray of liquid is formed and ejected from the nozzle opening. Although the dispersion method generally results in an improved coating application of the interior cylindrical wall of the pipeline, an inability to control the amount of bypass flow, and therefore control the maximum flow rate through the nozzle, erodes the effectiveness of the dispersion. The high velocity of product passing through the nozzle may stall the pig, resulting in pressure buildup behind the pig that causes the pig's speed to surge far above the speed at which it effectively disperses the liquid. Therefore, a need exists for a device to provide maximum speed reduction and prevent stalling and surging of the pig without reliance upon sophisticated control means.
Likewise, in the case of a cleaning type pig, optimum speed for effective cleaning may be significantly slower than the average flow in high-flow pipelines. A large amount of bypass may slow the pig and cause it to stall, especially as debris builds up in front of the pig. This stalling may result in pressure build-up and surging (as described above), resulting in high pig velocity and skating over portions of the debris. Again, a means to minimize the duration of stalling, pressure build-up, and surging is needed.